1. Field of the Invention
This invention relates to an organic electroluminescent (EL) element.
2. Background Art
Doping is one technique for producing light emission of any desired color from EL elements. It was reported in Jpn. J. Appl. Phys., 10, 527 (1971) to change emission color from blue to green by doping anthracene crystals with a minor amount of tetracene. With respect to organic thin film EL elements having a multilayer structure, it was reported in JP-A 264692/1988 to incorporate in a host material having a light emitting function a minor amount of a fluorescent dye capable of emitting light different from that of the host material in response to light emission from the host material as a dopant to form a light emitting layer, thereby changing the color of light emission from green to orange or red.
With respect to long wavelength light emission of yellow to red, known light emitting materials or dopant materials include laser dyes capable of red oscillation (EP 0281381), compounds capable of exciplex emission (JP-A 255788/1990), perylene compounds (JP-A 791/1991), coumarin compounds (JP-A 792/1991), dicyanomethylene compounds (JP-A 162481/1991), thioxanthene compounds (JP-A 177486/1991), mixtures of a conjugated polymer and an electron transporting compound (JP-A 73374/1994), squalirium compounds (JP-A 93257/1994), oxadiazole compounds (JP-A 136359/1994), oxynate derivatives (JP-A 145146/1994), and pyrene compounds (JP-A 240246/1994).
Other light emitting materials disclosed heretofore include condensed polycyclic aromatic compounds (JP-A 32966/1993 and 214334/1993). Also dopant materials proposed heretofore include various condensed polycyclic aromatic compounds (JP-A 258859/1993).
These light emitting systems, however, do not provide high luminance or stable light emitting performance. A further improvement in luminance or durability is thus desired.